There is currently no way to search for Gutenberg-ready themes on WordPress.com themes because there is no filter set up for this. However, the team said users should not any experience any issues with themes breaking with the new editor:

All existing themes should still work with Gutenberg. At worst styles in the editor might not exactly match styles on the site itself, and styling for individual blocks might cause conflicts if the theme treats that type of content in a specific way. But that is true of all WordPress themes, not just the ones on WordPress.com.

Users can activate any theme they want with Gutenberg. The new editor is not going to break any themes, but a theme does need to add support for users to take advantage of specific features like wide alignments and block color palettes. Gutenberg-ready themes also include editor styles to ensure a consistent editing experience between frontend and backend.

Automattic is also working to bring some of those updates from its current set of Gutenberg-ready themes to its free themes hosted on WordPress.org. The company has 109 themes in the directory, which have cumulatively been downloaded more than 17 million times. The majority of its more popular themes fall into the business category, such as Dara (10K active installs), Argent (10K), Edin (6K), and Karuna (5K). Several of these themes are already Gutenberg-ready with the code available on GitHub.

Storefront is by far Automattic’s most popular free theme on WordPress.org with 200,000+ installs and is well on its way towards being ready to support Gutenberg’s new features. Development towards this goal is happening on GitHub. Users can run beta versions of the Storefront theme ahead of time using the Storefront Beta Tester plugin.

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